Apparatus and method for directional solidification

ABSTRACT

Control of manufacture of individual directionally solidified articles is provided by an improved apparatus, an important feature of which is a two-chamber vacuum casting furnace including a pair of cooperating chill members, a first being mounted in an upper chamber and a second being movable with a casting mold. Heating means applies heat to develop a plurality of heating zones within the furnace. The method practiced involves removing heat from metal in the mold initially predominantly through the first chill member and then through both chill members which are diverging one from the other. One chill member removes heat through the base of the casting being solidified while the other removes heat through the lateral walls of the casting predominantly at the liquid-solid interface traversing the solidifying casting.

United States Patent Smashey Aug. 5, 1975 fiat/1.; (Av/(a4 (AA/760dPrimary ExaminerFrancis S. Husar Assistant Examiner-John E. RoethelAttorney, Agent, or FirmLee 1-1. Sachs; Derek P. Lawrence [5 7 ABSTRACTControl of manufacture of individual directionally solidified articlesis provided by an improved apparatus, an important feature of which is atwo-chamber vacuum casting furnace including a pair of cooperating chillmembers, a first being mounted in an upper chamber and a second beingmovable with a casting mold. Heating means applies heat to develop aplurality of heating zones within the furnace. The method practicedinvolves removing heat from metal in the mold initially predominantlythrough the first chill member and then through both chill members whichare diverging one from the other. One chill member removes heat throughthe base of the casting being solidified while the other removes heatthrough the lateral walls of the casting predominantly at theliquidsolid interface traversing the solidifying casting.

7 Claims, 1 Drawing Figure APPARATUS AND METHOD FOR DIRECTIONALSOLIDIFICATION The invention herein described was made in the course ofor under a contract, or a subcontract thereunder, with the United StatesDepartment of the Air Force.

BACKGROUND OF THE INVENTION This invention relates to casting of metalarticles and, more particularly, to the casting of metal articlesdirectionally solidfied to include an elongated grain structure.

The advantages of providing an elongated, directionally oriented grainstructure in a metal article through directional solidification includepredominantly a significant advantage in thermal fatigue life overconventionally cast structures having an equiaxed grain structure.However, because current apparatus and methods, which have been welldocumented in the literature, generally have evolved from vacuumprecision casting technology, relatively large, expensive and relativelydifficult to control furnace apparatus has been used. In addition, shellcluster molds for casting of a plurality of articles from a single batchof poured molten metal have generally been employed.

A key to the efficiency and rate of production of directionallysolidified articles is the control of heat transfer from the metalpoured into the mold, through the mold and into other furnace apparatus.In general, a mold is positioned on a chill plate through which heatpasses from the solidifying casting by conduction. If the mold iswithdrawn from the heated zone of a furnace as in the withdrawal method,heat transfer is enhanced by radiation into the unheated chamber intowhich the casting is drawn as the casting solidifies. However, accuratecontrol of the position of the liquidsolid interface at which the grainsare solidifying is important yet is difficult to achieve with reportedapparatus.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention to provide, for the manufacture of a single directionallysolidified article by the withdrawal method, an improved vacuum castingapparatus which includes improved chill members to more accuratelycontrol heat transfer from a mold chamber in which the article is beingsolidified.

Still another object is to provide such an apparatus including a furnacewith heating means disposed to develop a plurality of heating zones toprovide flexibility in heating desired portions of the furnace.

A further object is to provide an improved directional solidificationmethod in which heat after casting initially is removed predominantlythrough a chill member which first contacts molten metal poured into acasting mold and then, in addition, through a second chill member aboutthe mold lateral wall, the rate of withdrawal of the mold from theheated portion of the furnace being controlled to maintain the advancingliquid-solid interface preferable in the area of the top of the secondchill member.

These and other objects and advantages will be more clearly understoodfrom the following detailed description, the drawing and examples, allof which are in tended to be typical of rather than in any way limitingon the scope of the present invention.

In one more specific form, the present invention provides a vacuumcasting enclosure which includes an upper and a lower chamber along withmeans to apply heat to the upper chamber. A vacuum valve can connect thechambers. The upper chamber includes a base having an opening which,through the vacuum valve, connects the upper chamber to the lowerchamber and on which a first chill member is mounted. The first chillmember includes a vertical chill passage through the member to allowpassage of a mold through the chill member and through the base of theupper chamber. The lower chamber, which most conveniently includes anaccess port, also encloses a second, movable chill member including aportion which is sized to pass through the vertical chill passage of thefirst chill memher after passing through the base of the upper chamber.Such second chill member is adapted to carry a casting mold. Means areprovided to move the second chill member vertically between the twochambers. The apparatus also includes heating means which can becontrolled to apply heat at various rates as desired within the upperchamber.

One form of the withdrawal method of the present invention includesremoving heat from a molten metal filled mold initially at the moldbottom portion predominantly through a base chill member and then, asthe mold is withdrawn, additionally through a chill membercircumferentially disposed substantially about the mold and closelyadjacent lateral portions of the mold. The rate of withdrawal of themold from the heated upper chamber is controlled with the rate of heattransfer from the mold into the chill members to maintain theliquid-solid interface of a solidifying metal article within the mold inthe area of the top surface of the first chill member and generally justabove such surface. It should be understood that as used herein, theterm metal is intended to include metal alloys.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a partially sectional,partially diagrammatic view of one form of the apparatus involving thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Development of the withdrawalmethod for manufacturing directionally solidified articles eliminated anumber of problems which had existed in connection with earlierdeveloped methods. In general, the withdrawal method involves placing ahot ceramic shell cluster mold on a chill plate mounted on an elevatormechanism. As the solidification zone starts to move upward byconduction of heat to the chill plate, the mold is withdrawn from thehot zone of the furnace at a predetermined rate into an unheated chamberor portion of the furnace. Heat transfer by conduction through the chillplate at the base of the mold is then enhanced by radiation toward wallsof the unheated chamber. As the mold is withdrawn, the conductive paththrough the solidifying casting to the chill plate is increased to apoint at which its effectiveness is greatly reduced. Then radiation fromthe solid portion of the casting drawn into the unheated chamber is themechanism relied upon to maintain the thermal gradient at theliquid-solid interface within the casting mold. However, this basicmethod is lacking in its ability to maintain in a very precise manner amaximum thermal gradient at the solidifying interface throughout thetotal cycle. One reason is that a cluster mold, including cavities for aplurality of articles to be cast from a single pour of molten metal, isused. In addition, precise control of heat transfer from the mold hasnot been provided.

The furnace involved with the present invention provides capability forattaining such maximum thermal gradient through a combination of aplurality of chill members along with precise, selective heatapplication. The plurality of chill members includes one which duringwithdrawal closely surrounds a single mold. Another, on which the moldis mounted, constitutes a base chill plate movable with the mold andthrough which heat is conducted from the metal from which the article isbeing made.

The drawing shows one form of the apparatus in a partially sectional,partially diagrammatic view. The vacuum casting apparatus involving thepresent invention includes an enclosure shown in the embodiment of thedrawing to have an upper chamber shown generally at 10, a lower chambershown generally at 12 and a vacuum valve shown generally at 14connecting the upper and lower chambers. Associated with the upper andlower chambers are means to evacuate such chambers such as through portsat 16 and 18. Such means can, for example, include a common vacuum pump19, or individual pumps, to create a vacuum within the upper chamber andlower chamber as desired.

Within upper chamber is a furnace shown generally at 21, and includingheating means shown in the drawing to be three vertically stackedindividual resistance windings 20a, 20b and 200 as heat sources. Eachwinding is powered from a source of electrical energy and is controlledthrough a furnace control means 22 which can vary the power input toeach source. Control means 22 also can coordinate the rate of heatapplied by each heat source to the hollow interior of chamber 10 throughthe use of standard variable power control apparatus commerciallyavailable and well known in the electrical art. Although the heatingmeans which applies heat to the furnace in the upper chamber is shown asa resistance-wound three-part unit, it will be understood by thoseskilled in the art that a variety of means of applying and controllingheat might be used to accomplish the intended thermal control in thevarious zones involved.

One important feature of the present invention is that the furnace wallsformed by the heating means and which together define a furnace hollowinterior 35, are disposed closely adjacent and substantially enclose acasting mold lateral and top portions. This arrangement, along with theheating means, provides more accurate control of the metal within themold, particularly at the start of the method involved with the presentinvention. the furnace walls enclosing the heating elements generallyare of a ceramic material such as alumina.

Upper chamber 10 includes a base 24 having an opening 26 therethrough toenable communication between upper chamber 10 and lower chamber 12through vacuum valve 14. Mounted on base 24 is a circumferentiallydisposed first chill member 28 which includes a top surface 30 and avertical chill passage 32 through the first chill member and alignedwith upper chamber base opening 26. The first chill member is preferablymetal, for example, copper or a copper-base alloy, and preferablyincludes means diagrammatically represented as conduit 33 associatedwith a cooling fluid source (not shown) to circulate a cooling fluidthrough the chill, for example water, to enhance the heat transferthrough the first chill member. Such cooling means can be disposed ascooling coils within or around the chill in a manner well known in theart, for example, in connection with water-cooled heat transfer members.

In order to allow intercommunication between upper chamber 10 and lowerchamber 12, vacuum valve 14 includes a means 34 to operate vacuum valve14. Vacuum valve 14 and means 34 are of a type commercially available,well known and widely used in the vacuum furnace art involving multiplecompartment furnaces. Through the use of such a vacuum valve, upperchamber 10 can be environmentally isolated from lower chamber 12 tomaintain in upper chamber 10 a vacuum, once it has been developed there,while lower chamber 12 is used for loading and unloading molds beforeand after operation.

Lower chamber 12 has an access port 3 1 which includes a door 38 havingassociated vacuum sealing means 39. Access port 36 can be of anyconvenient shape, for example to accommodate loading or unloading of acasting mold, such as of ceramic shown gener ally at 40. Mold 40includes a foot or base 4], lateral walls 43 and top portion 45.

Shown in lower chamber 12 is a mold platform 42 operatively connectedwith a vertically operating elevator mechanism 44 adapted to raise andlower mold platform 42 toward and away from upper chamber 10. Mounted onmold platform 42 is a second chill member 46, movable with the moldplatform, and having a top surface 47 on which casting mold 40 ismounted. This is one example of means to provide relative movementbetween the chill members 28 and 46 and hence such movement between mold411 and first chill member 28.

Second chill member 46, sometimes referred to as a base, preferably ismetal and can be fluid cooled in a manner similar to the first chillmember. The second member is shaped to pass into vertical chill passage32 of first chill member 28 in upper chamber 10, for example, by havingits lateral wall 49 shaped to a slightly smaller configuration of chillpassage 32. Accordingly, elevator mechanism 44 has a vertical strokesufficient to raise second chill member 46 into vertical chill passage32, thus to enable positioning of casting mold 40 within upper chamber10, as is shown in phantom in the drawing. Elevator 44, which can be amachine screw type mechanism driven by a reversible rotating means suchas a reversible motor diagrammatically represented by arrows 48, ishoused within a jacket 50 including appropriate vacuum sealing means toisolate lower chamber 12 from the atmosphere.

Associated with rotating means 48 is an elevator control 52 capable ofinitiating and terminating the operation of rotating means 48 and, ifdesired its speed. In a more automated form of the present invention,elevator control 52 is coordinated with furnace control 22, in a mannerwhich will be described in more detail in connection with one form ofthe method associated with the present invention. This can beaccomplished through process control means 54, one principal function ofwhich is to time the heat applied in upper chamber 10 through furnacecontrol 22 with the rate of withdrawal of casting mold 40 from furnace21 through elevator control 52. In a still more automated form of theapparatus involving the present invention, coordinating process controlmeans 54 can initiate operation of means 34 to operate vacuum valve 14as a function of a signal from a pressure sensor 56 within lower chamber12 signalling control means 54 that an adequate vacuum has been providedwithin lower chamber 12 to enable opening of vacuum valve 14. Inaddition, control means 54 can be programmed to close valve 34 as afunction of the position of casting mold 40 being withdrawn from upperchamber and passing through vacuum valve 14. In one form, such sensingmeans can be a commercially available proximity switch 58 in lowerchamber 12 and a similar switch (not shown) in upper chamber 10 to sensethe position of mold 40 such as through mold platform 42. A furtherfunction which can be performed by coordinating control means 54 is toinitiate production of a vacuum, or to release the vacuum, within lowerchamber 12, for example as a function of the sealing of access port 36or of the mold position. For example, this can be accomplished through avalve 60 associated with lower chamber evacuation port 18 to provideevacuation of the lower chamber.

The vacuum casting furnace can be supported in a variety of ways, asthose skilled in the art will recognize. A support member 62 is showndiagrammatically in the drawing to represent support means. The locationof a single furnace or an arrangement of a plurality of such vacuumcasting furnaces, which with its controls each defines a furnace module,may suggest a particular support means most useful to one skilled in theart.

The close control for directional solidification provided by the presentinvention is accomplished in part by applying heat wiithin furnace 21 ata plurality of rates to accomplish different functions. For example,heat is applied to the interior of furnace 21 in amounts first to melt asolid metal charge and then to maintain the temperature of melted metalwithin casting mold 40 at a temperature greater than its meltingtemperature, except that perhaps for a relatively small area at the baseof the mold in which solidification is occurring. The present inventionis particularly adapted to use a self-casting mold, for example of thetype described in co-pending application Ser. No. 41 l ,927, filedconcurrently with this application. Therefore, one form of the methodassociated with the present invention requires heat application toachieve the highest temperature in the mold in the top zone of thefurnace, indicated at A, in order to bring about as rapid alloy chargemelting as possible. If desired, further variation of heat applicationwithin a zone such as top zone A can be provided for more selectivecontrol of charge melting. Because of the heat carried away by chillmember 30, heat application to the lower zone, indicated at C, isrelatively high compared with intermediate zone, indicated at B, inorder to maintain metal within the mold above its melting temperature,except below the liquid-solid in terface near the base of the mold atwhich directional solidification initially is occurring. Thus, thepresent invention includes the application of heat to the furnaceinterior at a plurality of rates to control more closely the casting andthen the directional solidification of the metal within the mold as themethod proceeds. As was mentioned before, the close control afforded bysuch variable application of heat in the zones described is enhanced bydisposing the furnace walls closely adjacent and substantially enclosingthe casting mold lateral and top portions.

After sufficient temperature is generated in zone A of the furnace tomelt the alloy charge and allow it to flow into the bottom part of themold, heat is removed from the mold initially through the base chillmember, indicated at 46 in the drawing. Chill member 46, at thebeginning of the method, is disposed within vertical chill passage 32 ofthe first chill member 28 shown in the drawing to be circumferentiallydisposed about and closely adjacent the path the mold traverses. Then,as the mold is withdrawn from furnace 21 as a result of the downwardmovement of elevator mechanism 44, heat is withdrawn through the basechill member 46 as well as through the chill member 28 which becomesdisposed substantially about a lateral portion or circumference of mold40. Practice of the method includes coordinating elevator control 52with furnace control 22 to maintain the liquid-solid interface of thedirectionally solidifying alloy within mold 40 in the vicinity of thetop surface 30 of the first or circumferentially disposed chill member28. This improved control of heat fiow through the practice of thepresent invention, employing the plurality of chill members divergingone from the other but disposed at those areas of the mold requiring theclosest heat flow control, eliminates casting defects such as strayequiaxed grains, freckles, misoriented grains and shrink.

Referring to the drawing, one form of the method involved in the presentinvention is practiced by first closing vacuum valve 14 while elevatormechanism 44 and platform 42 are disposed substantially as shown in thedrawing within lower chamber 12. A vacuum is then provided in upperchamber 10 and furnace 21 through upper chamber port 16 and vacuum pump19. A self-casting mold 40, including a solid metal charge in its upperporion, is secured to base or second chill member 46. Access door 38 isclosed and sealed after which lower chamber 12 is evacuated through port18 and a vacuum pump such as 19. As was mentioned before, the closingand sealing of door 38 can signal valve to initiate creation of thedesired vacuum in lower chamber 12. Such pressure level can be sensed bypressure sensor 56 which can then signal process control 54 to operatemeans 34 to open vacuum valve 14. Thereafter, process control 54 cansignal elevator control 52 to raise mold 40 to the position shown inphantom in the drawing as sensed by a proximity switch appropriatelylocated. In any event, this form of the method of the present inventionbasically includes providing an appropriate vacuum in lower chamber 12,opening vac uum valve 14 and elevating mold 40 into position withinfurnace 21.

With the mold in the position shown in phantom in the drawing, heatapplied at a high rate, primarily as a result of heating means 20a,raises the temperature of the solid metal charge in the top portion ofthe mold above its melting temperature. The molten charge then flowsdownwardly by gravity filling the lower portion of the mold in which anarticle is to be generated. At the same time, heat is applied throughheating means 20b, and at a higher rate through heating means 20c, tomaintain the cast charge above its melting point except in the area oftop surface 30 of the circumferentially disposed first chill member.Elevator control 52 is then activated to lower mold 40 from furnace 21.However, the rate of withdrawal is coordinated with the heat appliedthrough furnace control 22 to the various zones within furnace 21 tomaintain the liquid-solid interface of the directionally solidifyingmetal within the mold in the general area of the top surface 30 of thefirst chill member 28 circumferentially disposed about the withdrawingmold. In this way, the liquid-solid interface traverses the mold at aclosely controlled rate to provide a directionally solidified article ofimproved quality.

After complete withdrawal of mold 40 from furnace 21 in upper chamber 10into lower chamber 12, vacuum valve 14 is closed to maintain vacuum inupper chamber 10. Closing of valve 14, such as through means 34, can beaccomplished as a result of a signal, such as from a proximity switch,to process control 54 which directs such closure. Vacuum is thenreleased from lower chamber 12, such as by opening valve 60, manually oron signal from process control 54. Access door 38 is then opened, mold40 is then removed, and the apparatus is ready for another cycle.

Although the present invention has been described in connection withspecific examples and embodiments, it will be understood by thoseskilled in the art, the variations and modifications of which theinvention is capable within its broad scope.

What is claimed is:

1. In a method for directionally solidifying an article in a castingfurnace from a molten metal cast into a casting mold having a mold footand outer lateral walls, wherein heat is first removed from the moltenmetal through cooling means connected with the mold foot and then heatis removed concurrently through the mold foot and the lateral walls bycausing relative motion between the mold and a chill member to move asolidifying metal zone through the mold, the chill member having acentral opening defined by a chill wall and a chill top surface;

the improvement comprising, in combination, the

steps of:

providing the chill member with a chill wall and the casting mold withan outer lateral wall, each of said walls configured to be closelyadjacent one to the other during the relative motion between the moldand the chill member, with the chill wall circumferentially disposedabout the outer lateral wall, the lateral wall enclosing a singlearticle casting chamber communicating with the mold foot;

causing molten metal to be deposited in the casting chamber with themold foot closely adjacent the chill wall at the chill top surface;while at the same time,

applying a first amount of heat selectively toward the mold outerlateral wall in a first furnace area immediately above the chill topsurface to heat the wall when in the first area sufficiently to maintainthe metal in substantially the molten state; and concurrently,

applying a second amount of heat, less than the first amount butsufficient to maintain the metal in substantially the molten state,selectively toward the mold outer lateral wall when in a second furnacearea above the first area; while,

removing heat from the metal progressively during relative motionbetween the mold and the chill member through the means connected withthe mold foot and through the chill wall at a rate sufficient tosubstantially solidify the metal as it traverses adjacent the chill topsurface.

2. The method of claim 1, for use with a casting mold of theself-casting type having a top portion adapted to hold a solid metalcharge, in which:

the molten metal charge is deposited in the casting chamber by applyingconcurrently with the first and the second amounts of heat, a thirdamount of heat in a third furnace area toward the mold top portionholding the solid metal charge,

the third amount of heat being greater than the second amount and lessthan the first amount but sufficient to melt the solid metal charge.

3. Vacuum casting apparatus including:

walls defining an enclosure having an upper chamber and a lower chamber;

evacuating means to evacuate the upper and lower chambers;

isolating means to environmentally isolate the upper and lower chambersone from the other;

a first chill member having a chill passage therethrough and locatedwithin the upper chamber;

a second chill member within the enclosure sized to pass into the chillpassage and adapted to carry a casting mold;

means to move the second chill member between the lower chamber and thefirst chill passage; and

heating means comprising a plurality of elements in substantial verticalarray to heat the upper chamber;

the improvement wherein:

the heating means includes a vertically stacked plurality of separatelycontrolled heat sources, each positioned substantially vertically abovethe first chill member, the heat sources having interior surfacessubstantially aligned with each other and with the chill passage todefine a hollow furnace interior adapted to receive a casting mold witha single article casting cavity enclosed by lateral walls, the interiorsurfaces being positioned closely adjacent and substantially enclosingthe mold lateral walls dur ing operation;

the apparatus including furnace control means operatively connected witheach heat source and including means for applying heat concurrently at aplurality of rates and for varying the intensity electrical power toeach heat source independent of the other heat sources.

4. The apparatus of claim 3 in which the furnace control means isadapted to apply a relatively larger amount of electrical power to afirst of the heat sources adjacent the first chill member concurrentlywith the application of a relatively smaller amount of electrical powerto a heat source above the first heat source.

5. The apparatus of claim 3 which includes, in addition:

process control means to coordinate the rate of heat application to thehollow furnace interior through the furnace control means with the meansto move the second chill member between the lower chamber and the firstchill passage.

6. The apparatus of claim 5 in which the process control means isoperatively connected with the isolating means to coordinate movement ofthe isolating means with the furnace control means and the means to movethe second chill member.

7. The apparatus of claim 6 in which the process control means also isoperatively connected with the evacuating means to coordinate theevacuating means with the isolating means.

1. In a method for directionally solidifying an article in a castingfurnace from a molten metal cast into a casting mold having a mold footand outer lateral walls, wherein heat is first removed from the moltenmetal through cooling means connected with the mold foot and then heatis removed concurrently through the mold foot and the lateral walls bycausing relative motion between the mold and a chill member to move asolidifying metal zone through the mold, the chill member having acentral opening defined by a chill wall and a chill top surface; theimprovement comprising, in combination, the steps of: providing thechill member with a chill wall and the casting mold with an outerlateral wall, each of said walls configured to be closely adjacent oneto the other during the relative motion between the mold and the chillmember, with the chill wall circumferentially disposed about the outerlateral wall, the lateral wall enclosing a single article castingchamber communicating with the mold foot; causing molten metal to bedeposited in the casting chamber with the mold foot closely adjacent thechill wall at the chill top surface; while at the same time, applying afirst amount of heat selectively toward the mold outer lateral wall in afirst furnace area immediately above the chill top surface to heat thewall when in the first area sufficiently to maintain the metal insubstantially the molten state; and concurrently, applying a secondamount of heat, less than the first amount but sufficient to maintainthe metal in substantially the molten state, selectively toward the moldouter lateral wall when in a second furnace area above the first area;while, removing heat from the metal progressively during relative motionbetween the mold and the chill member through the means connected withthe mold foot and through the chill wall at a rate sufficient tosubstantially solidify the metal as it traverses adjacent the chill topsurface.
 2. The method of claim 1, for use with a casting mold of theself-casting type having a top portion adapted to hold a solid metalcharge, in which: the molten metal charge is deposited in the castingchamber by applying concurrently with the first and the second amountsof heat, a third amount of heat in a third furnace area toward the moldtop portion holding the solid metal charge, the third amount of heatbeing greater than the second amount and less than the first amount butsufficient to melt the solid metal charge.
 3. Vacuum casting apparatusincluding: walls defining an enclosure having an upper chamber and alower chamber; evacuating means to evacuate the upper and lowerchambers; isolating means to environmentally isolate the upper and lowerchambers one from the other; a first chill member having a chill passagetherethrough and located within the upper chamber; a second chill memberwithin the enclosure sized to pass into the chill passage and adapted tocarry a casting mold; means to move the second chill member between thelower chamber and the first chill passage; and heating means comprisinga plurality of elements in substantial vertical array to heat the upperchamber; the improvement wherein: the heating means includes avertically stacked plurality of separately controlled heat sources, eachpositioned substantially vertically above the first chill member, theheat sources having interior surfaces substantially aligned with eachother and with the chill passage to define a hollow furnace interioradapted to receive a casting mold with a single article casting cavityenclosed by lateral walls, the interior surfaces being positionedclosely adjacent and substantially enclosing the mold lateral wallsduring operation; the apparatus including furnace control meansoperatively connected with each heat source and including means forapplying heat concurrently at a plurality of rates and for varying theintensity electrical power to each heat source independent of the otherheat sources.
 4. The apparatus of claim 3 in which the furnace controlmeans is adapted to apply a relatively larger amount of electrical powerto a first of the heat sources adjacent the first chill memberconcurrently with the application of a relatively smaller amount ofelectrical power to a heat source above the first heat source.
 5. Theapparatus of claim 3 which includes, in addition: process control meansto coordinate the rate of heat application to the hollow furnaceinterior through the furnace control means with the means to move thesecond chill member between the lower chamber and the first chillpassage.
 6. The apparatus of claim 5 in which the process control meansis operatively connected with the isolating means to coordinate movementof the isolating means with the furnace control means and tHe means tomove the second chill member.
 7. The apparatus of claim 6 in which theprocess control means also is operatively connected with the evacuatingmeans to coordinate the evacuating means with the isolating means.